In the art of thermal ink-jet printing, it is known to provide a plurality of electrically resistive elements on a common substrate for the purpose of heating a corresponding plurality of ink volumes contained in adjacent ink reservoirs leading to the ink ejection and printing process. Using such an arrangement, the adjacent ink reservoirs are typically provided as cavities in a barrier layer attached to the substrate for properly isolating mechanical energy to predefined volumes of ink. The mechanical energy results from the conversion of electrical energy supplied to the resistive elements which creates a rapidly expanding vapor bubble in the ink above the resistive elements. Also, a plurality of ink ejection orifices are provided above these cavities in a nozzle plate and provide exit paths for ink during the printing process.
In the operation of thermal ink-jet printheads, it is necessary to provide a flow of ink to the thermal, or resistive, element causing ink drop ejection. This has been accomplished by manufacturing ink refill channels, or slots, in the substrate, ink barrier, or nozzle plate.
Current thermal ink-jet pen designs utilize a resistor multiplex pattern which allows the resistors to be "fired" at different times. Therefore, the resistors are offset spatially to compensate for this timing. These pens are fabricated by cutting the ink refill slot through a silicon substrate, which provides a vertical edge, or shelf, perpendicular to the print swath, while the resistors are staggered with respect to this edge, thereby creating different path lengths from the ink source or fill slot for each resistor.
The consequence of this design is that the entrance length (the distance from the edge of the shelf to the channel entrance on an individual chamber basis) varies from 61 .mu.m to 94 .mu.m, with the nominal shelf length of 40 .mu.m on one particular commercial thermal ink-jet pen. Currently, all chambers have a 90.degree. tapered fang residing between the slot and the channel. The line width frequency testing has shown that the refill speed varies between chambers, with the 61 .mu.m entrance length producing a "faster" chamber than the 94 .mu.m entrance length. Specifically, the nozzles with shortest entrance lengths are 350 Hz faster than those furthest from the slot.
The different path lengths offer varying resistance to ink flow and thus vary the time it takes to refill each resistor firing chamber. The chamber cannot be fired in a predictable manner until refill takes place. In addition, these varying resistances vary the damping of the chamber. If a chamber is over-damped, it is a slower structure than optimum and if under-damped, can cause nozzle instability resulting in spray, etc.
One possible solution is to etch the silicon shelf leading up to the inlet channel; see, e.g., application Ser. Nos. 08/009,151 and 08/009,181, both filed Jan. 25, 1993, and assigned to the same assignee as the present application. While that solution certainly provides a satisfactory result, it is nonetheless a costly process step.
Thus, there is a need to provide a mechanism for permitting all chambers to have the same refill speed, regardless of entrance length.